Fan-out package structure and methods for forming the same

ABSTRACT

A package includes a device die including a first plurality of metal pillars at a top surface of the device die. The package further includes a die stack including a plurality of dies bonded together, and a second plurality of metal pillars at a top surface of the die stack. One of the device die and the plurality of dies includes a semiconductor substrate and a through-via penetrating through the semiconductor substrate, A polymer region includes portions encircling the device die and the die stack, wherein a bottom surface of the polymer region is substantially level with a bottom surface of the device die and a bottom surface of the die stack. A top surface of the polymer region is level with top ends of the first and the second plurality of metal pillars. Redistribution lines are formed over the first and the second plurality of metal pillars.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation-in-part application of the followingcommonly-assigned U.S. patent application: application Ser. No.13/896,889, filed May 17, 2013, and entitled “Fan-Out Package Structureand Methods for Forming the Same,” which further claims the benefit ofthe following provisionally filed U.S. patent application: ApplicationSer. No. 61/754,362, filed Jan. 18, 2013, and entitled “Fan-Out PackageStructure and Methods for Forming the Same,” which applications arehereby incorporated herein by reference.

BACKGROUND

In integrated circuit applications, more and more functions areintegrated into products. For example, different functional elementssuch as 3G video elements, WiFi elements, Bluetooth elements, andaudio/video elements may need to be integrated together to form anapplication.

In conventional integration schemes, different components are bonded toan interposer, which is further bonded to a package substrate. Forexample, in mobile applications, a power management integrated circuitdie, a transceiver die, and a multi-layer ceramic capacitor may bebonded using this scheme. The resulting package is typically very thickand large in area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 9 illustrate cross-sectional views of intermediatestages in the manufacturing of a fan-out package structure in accordancewith some exemplary embodiments;

FIG. 10 illustrates a magnified cross-sectional view of a die stack inaccordance with some embodiments;

FIG. 11 illustrates a magnified cross-sectional view of a device die inaccordance with some embodiments; and

FIG. 12 illustrates an exemplary process flow for forming a packagestructure in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A fan-out package structure and the methods of forming the same areprovided in accordance with various exemplary embodiments. Theintermediate stages of forming the fan-out package structure areillustrated. The variations of the embodiments are discussed. Throughoutthe various views and illustrative embodiments, like reference numbersare used to designate like elements.

FIGS. 1 through 9 illustrate the cross-sectional views of intermediatestages in the formation of an interconnect structure in accordance withsome embodiments. The steps shown in FIG. 1 through 9 are alsoillustrated schematically in the process flow shown in FIG. 12. In thesubsequent discussion, the process steps shown in FIGS. 1 through 9 arediscussed referring to the process steps in FIG. 12.

FIG. 1 illustrates carrier 20 and release layer 22 formed on carrier 20.Carrier 20 may be a glass carrier, a ceramic carrier, or the like.Carrier 20 may have a round top-view shape and may have the size of asilicon wafer, as shown in FIG. 2B. Release layer 22 may be formed of apolymer-based material, which can be removed along with carrier 20 fromthe overlying structures formed in subsequent steps. In accordance withsome embodiments of the present disclosure, release layer 22 is formedof an epoxy-based thermal-release material. Release layer 22 may also bereferred to as a Light-To-Heat Conversion (LTHC) layer in someembodiments, which is capable of releasing the structures formed thereonunder the heat of the light. For example, release layer 22 may be formedof an Ultra-Violet (UV) glue. Release layer 22 may be dispensed as aliquid and cured. In alternative embodiments, release layer 22 is alaminate film and is laminated onto carrier 20. The top surface ofrelease layer 22 is coplanar and has a high degree of co-planarity.

FIGS. 2A and 2B illustrate the placement of device die(s) 24 and diestacks 124 and 224, wherein FIG. 2A illustrates a cross-sectional view,and FIG. 2B illustrates a top view. Device die 24 and die stacks 124 and224 are placed over release layer 22. In some embodiments, device die 24and die stacks 124 and 224 are directly attached to release layer 22,which is adhered to carrier 20. In alternative embodiments, each ofdevice die 24 and die stacks 124 and 224 is attached to release layer 22through Die-Attach Film (DAF) 25, which is an adhesive layer. DAFs 25are illustrated using dashed lines to indicate that they may or may notbe used. In subsequent figures, DAFs 25 are not illustrated, althoughthey may also be formed. Device die 24 may be a logic device dieincluding logic transistors therein. In some exemplary embodiments,device die 24 is a Central Processing Unit (CPU) die. In otherembodiments, device die 24 is a Graphics Processing Unit (GPU) die.Device die 24 may include a semiconductor substrate and active devices(such as transistors, not shown) formed on a surface of thesemiconductor substrate.

Each of die stacks 124 and 224 may include a plurality of memory dies324 bonded together to form a memory stack. Die stacks 124 and 224 mayinclude Dynamic Random Access Memory (DRAM) dies, Static Random AccessMemory (SRAM) dies, or memory dies formed of other types of memories. Insome embodiments, memory dies 324 are pre-bonded (for example, throughsolder bonding, direct metal-to-metal bonding, or the like) to form diestacks 124 and 224, and then the bonded die stacks 124 and 224 areplaced over carrier 20. In some embodiments, die stacks 124 and 224 areformed of a same type of memory (such as SRAM or DRAM). In alternativeembodiments, die stacks 124 and 224 are the stacks of different types ofmemories.

Electrical connectors 26 are formed as the top portions of device die 24and die stacks 124 and 224, and are electrically coupled to the devicesin device die 24 and die stacks 124 and 224. In some embodiments,electrical connectors 26 include metal pillars 26 (such as copperpillars), which may be pre-formed before device die 24 and die stacks124 and 224 are placed over carrier 20. Metal pillars 26 may besolder-free, and may comprise vertical sidewalls. In some embodiments,dielectric layers 27 are formed at the top surfaces of device die 24 anddie stacks 124 and 224, with metal pillars 26 having at least lowerportions, or entireties, in dielectric layer 27. The top surfaces ofdielectric layers 27 may also be substantially level with the top endsof metal pillars 26. Dielectric layers 27 may comprise polyimide,polybenzoxazole (PBO), an oxide layer, a nitride layer, or multi-layersthereof. Alternatively, dielectric layers 27 are not formed, and metalpillars 26 protrude above the remaining portions of device die 24 anddie stacks 124 and 224. In subsequently illustrated drawings, dielectriclayers 27 are not illustrated, although they may also exist in someembodiments. The thicknesses of device die 24, die stacks 124 and 224,and the heights of metal pillars 26 are controlled so that the top endsof metal pillars 26 of device die 24 are substantially level with thetop ends of metal pillars 26 of die stacks 124 and 224. Furthermore,since device die 24 and die stacks 124 and 224 are placed over carrier20 (for example, on adhesive 22), the back surfaces of device die 24 anddie stacks 124 and 224 are level with each other.

FIG. 10 illustrates a magnified view of die stack 124 or 224 (referredto as 124/224 hereinafter) in accordance with some embodiments of thepresent disclosure. Each of device dies 324 (including 324A, 324B, 324C,and 324D) in die stack 124/224 includes semiconductor substrate 304,wherein the active devices 305 such as transistors are formed at asurface of semiconductor substrate 304. In some embodiments,semiconductor substrate 304 is a crystalline silicon substrate. Inalternative embodiments, semiconductor substrate 304 includes anothersemiconductor material such as germanium, silicon germanium, a III-Vcompound semiconductor material, or the like. Metal lines and vias (notshown) are formed in interconnect structures 326 of device dies 324 tointerconnect the integrated circuit devices in device dies 324.

Through-vias (sometimes referred to as through-silicon vias orthrough-semiconductor vias) 306 are formed to penetrate throughsemiconductor substrates 304. Electrical connectors 308 are formed onthe top surfaces of device dies 324. Electrical connectors 310 mayfurther be formed on the bottom surfaces of device dies 324. Electricalconnectors 308 and 310 may be metal pads, metal pillars, or the like.Electrical connectors 308 are electrically coupled to the respectiveelectrical connectors 310 through through-vias 306. Furthermore, theintegrated circuits 305 in device dies 324 and electrical connectors 308may be electrically connected to electrical connectors 310 in devicedies 324.

As shown in FIG. 10, device dies 324 are bonded together to form diestack 124/224. In some embodiments, the bonding is through solderregions 314. In accordance with alternative embodiments, the bonding maybe direct metal-to-metal bonding without using solder. In accordancewith some embodiments of the present disclosure, device dies 324 areidentical to each other. In these embodiments, device dies 324 may beformed using identical process steps, wherein the different referencenumerals 324A, 324B, 324C, and 324D are used to indicate that they areat different levels in die stack 124/224. In alternative embodiments,device dies 324 have different structures including different circuitsand/or different metal routing, etc.

In some embodiments, underfill 312 is dispensed into the gaps betweenthe stacked device dies 324. Underfill 312 is then cured, for example,in a thermal curing process. In alternative embodiments, no underfill isdispensed, and the gaps between device dies 324 may be filled by moldingmaterial 40 (FIG. 3) in the subsequent molding step.

FIG. 11 illustrates a magnified view of device die 24 in accordance withsome embodiments of the present disclosure. Device die 24 may or may notinclude through-vias in the respective semiconductor substrate.Through-vias are formed when another package is to be bonded to theresulting package 48 (FIG. 8) from the top side of package 48. FIG. 11schematically illustrates device die 24 with through-vias 406 beingformed. In these embodiments, device die 24 includes semiconductorsubstrate 404, wherein the active devices 405 such as transistors areformed at a surface of semiconductor substrate 404. Semiconductorsubstrate 404 may be a crystalline silicon substrate, and/or may includegermanium, silicon germanium, a III-V compound semiconductor material,or the like.

Metal lines and vias 409 are formed in the interconnect structures 426of device die 24 to interconnect integrated circuit devices 405 indevice die 24. Through-vias 406 are formed to penetrate throughsemiconductor substrate 404. Metal pillars 26 are formed on the topsurface of device die 24, wherein metal pillars 26 may be embedded indielectric layer 27 in some embodiments. In alternative embodiments,metal pillars 26 protrude above the rest of device die 24. Electricalconnectors 410 may be formed at the bottom surfaces of device dies 324in some embodiments. Electrical connectors 410 may be metal pads, metalpillars, or the like, and may or may not include solder regions. Inaccordance with other embodiments, through-vias 406 and electricalconnectors 410 are not formed. Metal pillars 26 are electrically coupledto electrical connectors 410 through through-vias 406. Furthermore, theintegrated circuits in device die 24 and electrical connectors 408 areelectrically connected to electrical connectors 410 in device die 24.

FIG. 2B illustrates a top view of the structure in FIG. 2A. Thecross-sectional view in FIG. 2A is obtained from a plane crossing line2A-2A in FIG. 2B. Furthermore, device die 24 and die stacks 124 and 224are shown in the same plane in FIG. 2A for clarity, although they arenot necessarily in the same plane, as shown in FIG. 2B, for example. Insome embodiments, the placement of device die 24 and die stacks 124 and224 is at the wafer level, and hence there are a plurality of devicedies 24 and a plurality of die stacks 124 and 224 placed over carrier20. FIG. 2B illustrates that carrier 20 has a round top-view shape. Inalternative embodiments, carrier 20 have a rectangular top-view shape,and device die 24 and die stacks 124 and 224 may be laid out as anarray. In FIG. 2B, the rectangles (not marked) encircling each groups ofdevice die 24 and die stacks 124 and 224 mark the boundaries of therespective packages 48 (FIG. 7), which packages are formed in subsequentsteps.

Referring to FIG. 3, molding material 40 is dispensed and molded ondevice die 24 and die stacks 124 and 224. Molding material 40 fills thegaps between device die 24 and die stacks 124 and 224, and may be incontact with adhesive layer 22. Furthermore, molding material 40 may befilled into the gaps between metal pillars 26 if dielectric layers 27(FIG. 2) are not formed. Molding material 40 comprises a polymer in someembodiments. For example, molding material 40 may include a moldingcompound, a molding underfill, an epoxy, or a resin. The top surface ofmolding material 40 is higher than the top ends of metal pillars 26. Thebottom surface of molding material 40 is level with the back surfaces ofdevice die 24 and die stacks 124 and 224. After being dispensed, moldingmaterial 40 is cured.

Next, a planarization step, which may be a grinding step or a ChemicalMechanical Polish (CMP), is performed to thin molding material 40, untilmetal pillars 26 are exposed. The resulting structure is shown in FIG.4A. The top ends 26A of metal pillars 26 in device die 24 and die stacks124 and 224 are level with each other, and are level with top surface40A of molding material 40. In some embodiments in which no dielectriclayer 27 (FIG. 2) is formed, molding material 40 encircles, and is incontact with, each of metal pillars 26, as shown in FIG. 4A.

In alternative embodiments, as shown in FIG. 4B, dielectric layers 27are formed as the top surface layers of device die 24 and/or die stacks124/224, the top ends 26A of metal pillars 26 are level with each other,and are substantially level with the surfaces 27A of dielectric layers27 and top surface 40A of molding material 40.

In FIGS. 4A and 4B, the top surfaces of metal pillars 26 of device die24 and die stacks 124/224 are coplanar, and are coplanar with the topsurfaces of dielectric layers 27 (FIG. 4B, if any) and molding material40. Metal pillars 26 of device die 24, however, may have the same heightas or different heights from the height of the metal pillars 26 in diestacks 124/224. Accordingly, the bottom ends of metal pillars 26 ofdevice die 24 may or may not be coplanar with the bottom ends of themetal pillars 26 in die stacks 124/224.

Next, referring to FIG. 5, Redistribution Lines (RDLs) 42 are formedover molding material 40. RDLs 42 are also electrically connected to,and may interconnect, metal pillars 26. RDLs 42 are formed in dielectriclayers 44. There may be one, two, three, or more redistribution layer,each including a plurality of RDLs 42 that is at the same level. RDLs 42further include vias that interconnect the RDLs in neighboringredistribution layers. The RDLs 42 in the bottom redistribution layerand the respective dielectric layer 44 have bottom surfaces in contactwith the top ends of metal pillars 26 and the top surface of moldingmaterial 40. In some embodiments, RDLs 42 are formed by forming andpatterning dielectric layers 44, and forming RDLs 42 in the openings inthe patterned dielectric layers 44. In alternative embodiments, RDLs 42are formed by depositing metal layers, patterning the metal layers, andfilling the gaps between RDLs 42 with dielectric layers 44. In yetalternative embodiments, RDLs 42 and dielectric layers 44 are formedusing damascene processes. RDLs 42 may comprise copper, nickel,palladium, aluminum, tungsten, or the like. Dielectric layers 44 maycomprise photo-sensitive materials such as polyimide, PBO, or the like,which may be patterned without using additional photo resists. In someembodiments, all of dielectric layers 44 are formed using polymers suchas photo-sensitive materials. Dielectric layers 44 may also be formed ofa non-organic material or materials such as oxides and/or nitrides. RLDs42 and dielectric layers 44 are in combination referred to as interposer45 throughout the description. In accordance with the embodiments of thepresent disclosure, interposer 45 is formed starting from moldingmaterial 40, device die 24, and die stacks 124 and 224, which incombination act as a wafer having enough thickness and strength tosupport the formation of interposer 45. As a result, interposer 45 maybe very thin, for example, with a thickness smaller than about 50 μmwithout the concern that it may break during its formation and thesubsequent handling.

The bottom layer of dielectric layers 44 is in contact with the topsurface of molding material 40. Furthermore, the metal traces (RDLs) 42in the bottom RDL layer are in contact with the top surface of moldingmaterial 40 (and dielectric layers 27, if any), wherein no adhesive isdisposed between molding material 40 and the overlying dielectric layer44 and RDLs 42.

FIG. 5 further illustrates the formation of top dielectric layer 44(denoted as 44A), and the formation of openings 47 in top dielectriclayer 44A. The top dielectric layer 44A may also be formed of a polymersuch as PBO, polyimide, or the like. Openings 47 may be formed, forexample through laser drill, light-exposure and developing, or the like.The metal pads that are parts of the top RDLs 42 are exposed to openings47.

FIG. 6 illustrates the formation of electrical connectors 46 inaccordance with some exemplary embodiments. The formation of connectors46 may include placing solder balls on the exposed pad portions of RDLs42, and then reflowing the solder balls. In alternative embodiments, theformation of connectors 46 includes performing a plating step to formsolder regions over the pad portions of RDLs 42, and then reflowing thesolder regions. Connectors 46 may also include metal pillars, or metalpillars and solder caps, which may also be formed through plating.Throughout the description, the combined structure including device die24, die stacks 124 and 224, molding material 40, and the overlying RDLs42 and dielectric layers 44 is referred to as package 48 hereinafter.Package 48 is a part of a wafer 148 that includes a plurality ofpackages 48.

Referring to FIGS. 7, dicing tape 50 is attached to package 48 and therespective wafer 148, wherein carrier 20 and dicing tape 50 are on theopposite sides of package 48. Next, carrier 20 is detached from package48, and release layer 22 is removed. When release layer 22 is formed ofLTHC, release layer 22 decomposes under the heat of light, so thatcarrier 20 can be removed. For example, when release layer 22 is formedof the UV glue, release layer 22 may be exposed to UV light. Theresulting structure is shown in FIG. 8.

Further referring to FIG. 8, wafer 148 is sawed apart along scribe lines52 to separate wafer 148 into a plurality of packages 48. Each ofpackages 48 may include device die 24 and die stacks 124 and 224,molding material 40, and a piece of interposer 45 that includes RDLs 42and dielectric layers 44. As a result of the sawing, in the resultingpackages 48, the edges of dielectric layers 44 are aligned to therespective edges of molding material 40.

FIG. 9 illustrates the bonding and/or the attachment of package 48 toother package components. In some embodiments, connectors 46 are used tobond package 48 to another package component 58, which is a PrintedCircuit Board (PCB) in some exemplary embodiments. In some embodiments,no additional interposer and package substrate are bonded betweenpackage 48 and PCB 58. Interposer 45, which is built in package 48, isused to electrically couple device die 24 and device stacks 124 and 224to package component 58. In alternative embodiments, package 48 isbonded to an additional package substrate (not shown), which is furtherbonded to a PCB.

FIG. 9 also illustrates that the back surface of package 48 is attachedto heat spreader 54. In some embodiments, thermal tape (or ThermalInterface Material (TIM)) 56, which has a thermal conductivity higherthan the thermal conductivity of typical glues, is used to attach heatspreader 54 to package 48. Accordingly, the heat generated in device die24 and device stacks 124 and 224 may be dissipated to heat spreader 54.

FIG. 12 schematically illustrates the process flow 500 for the processesin FIGS. 1 through 9. The process flow is briefly discussed herein. Thedetails of the process flow may be found in the corresponding discussionof FIGS. 1 through 9. In step 502, device die 24 and die stacks 124 and224 are placed over release layer 22 and carrier 20, as shown in FIGS.2A and 2B. In step 504 of the process flow in FIG. 12, device dies 24and die stacks 124 and 224 are molded in molding material 40, and therespective formation process is illustrated in FIG. 3. In step 506 ofthe process flow in FIG. 12, a planarization such as a grinding processis performed to expose the metal pillars 26 of device die 24 and diestacks 124 and 224, and the respective formation process is illustratedin FIGS. 4A and 4B. In step 508 of the process flow in FIG. 12, RDLs 42and electrical connectors 46 are formed to connect to device die 24 anddie stacks 124 and 224, and the respective formation process isillustrated in FIGS. 5 and 6. In step 510 of the process flow in FIG.12, a die-saw process is performed to saw the structure formed inpreceding formation processes into packages, and the respectiveformation process is illustrated in FIGS. 7 and 8. In step 512 of theprocess flow in FIG. 12, the resulting package is further bonded toother package components and heat sinks, and the respective formationprocess is illustrated in FIG. 9.

The embodiments of the present disclosure have some advantageousfeatures. In accordance with the embodiments of the present disclosure,interposer 45 is built over the device die and device stacks after thedevice die and device stacks are molded. This is different from theconventional interposers that are manufactured first, and then bondedwith device dies and/or device stacks. In the process for forming thepackages in accordance with some embodiments, a molding material, adevice die, and/or a device stack act as the carrier for forminginterposer 45. Since interposer 45 does not need to be separated fromthe carrier (the molding compound and the dies molded therein), it canbe made very thin without the concern that it may break in thesubsequent handling. The thickness of the resulting package is hencesignificantly reduced.

In accordance with some embodiments, a package includes a device dieincluding a first plurality of metal pillars at a top surface of thedevice die. The package further includes a die stack including aplurality of dies bonded together, and a second plurality of metalpillars at a top surface of the die stack. One of the device die and theplurality of dies includes a semiconductor substrate and a through-viapenetrating through the semiconductor substrate. A polymer regionincludes portions encircling the device die and the die stack, wherein abottom surface of the polymer region is substantially level with abottom surface of the device die and a bottom surface of the die stack.A top surface of the polymer region is level with top ends of the firstplurality of metal pillars and top ends of the second plurality of metalpillars. Redistribution lines are formed over and electrically coupledto the first and the second plurality of metal pillars.

In accordance with other embodiments, a package includes a device diehaving a first plurality of metal pillars at a top surface of the devicedie. A die stack includes a plurality of dies bonded together. A secondplurality of metal pillars is at a top surface of the die stack. Thepackage further includes a molding material encircling the device dieand the die stack, and a dielectric layer over the molding material. Thedielectric layer includes a bottom surface contacting a top surface ofthe molding material, and redistribution lines in the dielectric layer.The bottom surfaces of the redistribution lines are in contact with thetop surface of the molding material. The redistribution lines are overand electrically coupled to the first and the second plurality of metalpillars.

In accordance with yet other embodiments, a package includes a devicedie including a first plurality of metal pillars at a top surface of thedevice die. The package further includes a die stack, which includes aplurality of dies bonded together. The plurality of dies hassemiconductor substrates and through-vias penetrating through therespective semiconductor substrates. A second plurality of metal pillarsis at a top surface of the die stack, wherein the second plurality ofmetal pillars is electrically coupled to the through-vias. A polymerregion molds the device die and the die stack therein, wherein a topsurface of the polymer region, top ends of the first plurality of metalpillars, and top ends of the second plurality of metal pillars form afirst planar surface. A dielectric layer is disposed over the polymerregion, wherein edges of the dielectric layer are aligned to respectiveedges of the polymer region. Redistribution lines are disposed in thedielectric layer. The redistribution lines are electrically coupled tothe first and the second plurality of metal pillars, and wherein bottomsurfaces of the redistribution lines and the dielectric layer form asecond planar surface in contact with the first planar surface.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a device die comprising afirst plurality of metal pillars at a top surface of the device die; adie stack comprising: a plurality of dies bonded together, wherein oneof the device die and the plurality of dies comprises a semiconductorsubstrate and a through-via penetrating through the semiconductorsubstrate; and a second plurality of metal pillars at a top surface ofthe die stack; a polymer region comprising first portions encircling thedevice die and the die stack, wherein a bottom surface of the polymerregion is substantially level with a bottom surface of the device dieand a bottom surface of the die stack, and wherein a top surface of thepolymer region is level with top ends of the first plurality of metalpillars and top ends of the second plurality of metal pillars; andredistribution lines over and electrically coupled to the first and thesecond plurality of metal pillars.
 2. The package of claim 1 furthercomprising a dielectric layer, with the redistribution lines in thedielectric layer, wherein a bottom surface of the dielectric layer islevel with bottom surfaces of the redistribution lines, and wherein thebottom surface of the dielectric layer is in contact with the topsurface of the polymer region.
 3. The package of claim 2, wherein thedielectric layer comprises a photo-sensitive material.
 4. The package ofclaim 2, wherein the polymer region comprises a second portion extendinginto spaces between the first plurality of metal pillars, and whereinthe second portion comprises a top surface in contact with thedielectric layer.
 5. The package of claim 4, wherein the second portionof the polymer region is further in contact with each of the firstplurality of metal pillars.
 6. The package of claim 1, wherein die stackcomprises a plurality of memory dies.
 7. The package of claim 1 furthercomprising an additional dielectric layer as a top portion of the devicedie, wherein the first plurality of metal pillars is in the additionaldielectric layer, with a top surface of the additional dielectric layerlevel with top ends of the first plurality of metal pillars, and whereinedges of the additional dielectric layer are aligned to respective edgesof a bottom portion of the device die.
 8. The package of claim 1 whereinall dielectric layers in the package and over the polymer region have atotal thickness smaller than about 50 μm.
 9. A package comprising: adevice die comprising a first plurality of metal pillars at a topsurface of the device die; a die stack comprising: a plurality of diesbonded together; and a second plurality of metal pillars at a topsurface of the die stack; a molding material encircling the device dieand the die stack; a dielectric layer over the molding material, whereinthe dielectric layer comprises a bottom surface contacting a top surfaceof the molding material; and redistribution lines in the dielectriclayer, wherein bottom surfaces of the redistribution lines are incontact with the top surface of the molding material, and theredistribution lines are over and electrically coupled to the first andthe second plurality of metal pillars.
 10. The package of claim 9,wherein one of the bottom surfaces of the redistribution lines isfurther in contact with a top surface of one of the first and the secondplurality of metal pillars.
 11. The package of claim 9, wherein a bottomsurface of the molding material is substantially coplanar with a bottomsurface of the device die and a bottom surface of the die stack, andwherein a top surface of the molding material is coplanar with first topends of the first plurality of metal pillars and second top ends of thesecond plurality of metal pillars.
 12. The package of claim 9, whereinone of the plurality of dies in the die stack comprises: a semiconductorsubstrate; and through-vias penetrating through the semiconductorsubstrate.
 13. The package of claim 9, wherein the device die comprises:a semiconductor substrate; and through-vias penetrating through thesemiconductor substrate.
 14. A package comprising: a device diecomprising a first plurality of metal pillars at a top surface of thedevice die; a die stack comprising: a plurality of dies bonded together,wherein the plurality of dies comprises semiconductor substrates andthrough-vias penetrating through the respective semiconductorsubstrates; and a second plurality of metal pillars at a top surface ofthe die stack, wherein the second plurality of metal pillars iselectrically coupled to the through-vias; a polymer region molding thedevice die and the die stack therein, wherein a top surface of thepolymer region, top ends of the first plurality of metal pillars, andtop ends of the second plurality of metal pillars form a first planarsurface; a dielectric layer over the polymer region, wherein edges ofthe dielectric layer are aligned to respective edges of the polymerregion; and redistribution lines in the dielectric layer, wherein theredistribution lines are electrically coupled to the first and thesecond plurality of metal pillars, and wherein bottom surfaces of theredistribution lines and the dielectric layer form a second planarsurface in contact with the first planar surface.
 15. The package ofclaim 14, wherein the polymer region comprises a molding compound. 16.The package of claim 14, wherein the dielectric layer comprises aphoto-sensitive material.
 17. The package of claim 14, wherein a bottomsurface of the device die is level with a bottom surface of the polymerregion and a bottom surface of the die stack.
 18. The package of claim14 further comprising solder regions electrically coupled to the firstand the second plurality of metal pillars, wherein the solder regionsare bonded to a Printed Circuit Board (PCB), and wherein the PCB and thedevice die are on opposite sides of the dielectric layer.
 19. Thepackage of claim 14, wherein the polymer region extends into firstspaces between the first plurality of metal pillars and second spacesbetween the second plurality of metal pillars.
 20. The package of claim19 further comprising an additional dielectric layer as a top portion ofthe device die, wherein the first plurality of metal pillars is in theadditional dielectric layer, with a top surface of the additionaldielectric layer level with top ends of the first plurality of metalpillars, and wherein edges of the additional dielectric layer arealigned to respective edges of a bottom portion of the device die.